So now the blame game begins – whose fault was it that Labour and the Liberal Democrats failed to strike a deal? Shimmering on the horizon for an evanescent moment was the chance of a progressive alliance, a rocket about to take off, shooting for the moon. The mission would end the old dysfunctional split between two centre-left parties and bring together a social democratic majority of voters in a country that is essentially not Conservative. So who blocked it?

All day today the heat-resistant tiles began to fall off. Then it crashed back to earth. The truth of who offered what, with malicious or good intent, will be raked over for years to come with claim and counter-claim about who really wanted it and who was just pretending as a face-saver. Labour says Nick Clegg was always going to march his troops into Cameron's camp but needed cover for his left flank.

Lib Dems say Labour only dallied at alliance, relishing the prospect of branding Clegg and his cohorts as neo-conservative cutters at the next election. Bystanders can only look at the wreckage.

Wide eyed and innocent, Labour say a cornucopia of temptations was on the table. Labour's negotiating team say they offered generously and in good faith but the Lib Dems walked away, just as they had always intended. Away went ID cards, the DNA database, the third runway. Promised was the Lib Dems' £10,000 income tax threshold, their mansion tax, their tax on top pensions, fixed-term parliaments and a referendum on the alternative vote reform (AV). One sticking point was the Lib Dem demand to have AV now, with a later referendum to confirm it; Labour thought the voters wouldn't wear a pre-emptive bill. The Lib Dems wanted AV plus, a more proportional system added as a second option to a referendum, but Labour feared its own side would sink it. Labour failed to trump the Tories by offering real proportional representation – but the Conservatives offered nothing better.

In that display of promises, surely, was a shining possibility of a progressive government to keep the common Conservative enemy out for another generation. If only Labour had offered such radicalism in its own manifesto in the first place.

So what made the Lib Dems sniff at the feast and walk away? The company at the table was not enticing, as the worst of the old Labour party, the knuckle-dragging neanderthal tendency, emerged to roaring opposition to the guests. David Blunkett, John Reid, Jack Straw, Diane Abbott, now unleashed from government, reminded the world how backward, how unprogressive, tribal and sectarian much of the People's Party still is.

So precarious would have been the rainbow coalition that a few Kate Hoey or Frank Field mavericks crossing the floor might have been enough to bring the government down. Douglas Alexander announcing they could not work with Alex Salmond was another death blow.

When Andy Burnham, human weather vane, came out as the first cabinet minister against the deal, it was plain he scented too many in the party were viscerally opposed. How do you make a knife-edge deal that requires every single last member of a tricky coalition to sign their name in blood with these dark-age denizens of the Labour party?

Or maybe, as the Lib Dems claim, they were distinctly chilly. It hardly matters. If even the Labour party is not united, forget a coalition. Those seeking a shred of comfort from this debacle will at least note the talks with Labour gave the Lib Dems strong ammunition to arm-twist better policies out of Cameron.

The unions argued strongly for the progressive alliance – despite Nick Clegg's ill-advised dismissal of them as "vested interests". They are by trade deal-makers. By every sinew of their history supporting whatever keeps Conservatives from office, knowing their members stand to suffer most along with the most vulnerable.

Not for them the self-indulgent frivolity of those who yearn for the luxury of opposition to "rethink" and "regroup". Last time they "regrouped" for 18 long years.

Some in Labour think this is their D-day, the start of their next victory – but it's their Dunkirk.